A speaker provided in an audio system and the like is a form of electric transducer which converts an electric signal (electric energy) received from an amplifier to an acoustic signal (acoustic energy). Speakers fall into certain broad categories based on the operating principle: dynamic speaker, electrostatic speaker, piezoelectric speaker, discharge speaker, electromagnetic speaker, for example. Among these, the currently predominating type is the dynamic speaker which has a high performance and additionally meets various requirements such as those relating to sound quality. An example of the dynamic speakers will be described below in detail.
As illustrated in FIG. 1(a), a speaker disclosed in, for example, Patent Document 1 includes an edge 102 connecting the periphery of a diaphragm 101 to a yoke 103 which holds the diaphragm 101 in place through the edge 102. A magnet 104 is disposed on the upper face of the approximately U-shaped cross-section yoke 103. A top plate 105 is provided on the upper face of the magnet 104. The yoke 103, the magnet 104 and the top plate 105 form a magnetic circuit 106. The periphery end of the yoke 103 is bent in the direction of the top plate 105. A voice coil 107 is disposed in the magnetic gap created between the periphery end of the yoke 103 and the top plate 105. The magnetic circuit 106 has a structure in which the magnet 104 is disposed inside the voice coil 107, and is thus called an inner-magnet-type magnetic circuit. Hereinafter, the surface of the diaphragm on the acoustic radiation side is referred to as the front face and the surface of the diaphragm facing the magnetic circuit is referred to as the rear face. In addition, with regard to the magnet, the yoke and the plate, the faces facing the diaphragm are referred to as the upper faces and the faces on the opposite side to the diaphragm are referred to as the lower faces.
On the other hand, Patent Document 2 discloses a speaker comprising an outer-magnet-type magnetic circuit having the magnet disposed outside the voice coil. As illustrated in FIG. 1(b), the speaker has a ring-shaped magnet 14 mounted on a yoke 13 on which a pole 11 and an engagement step 12a are formed. A plate 15 having an engagement protrusion 15b is mounted on the top of the magnet 14. A dome-shaped diaphragm 16 is provided above the pole 11 of the yoke 13. A U-shaped cross-sectional voice coil bobbin 17 is provided on the outer rim of the diaphragm 16, and is supported by an edge 18. The diaphragm 16, the voice coil bobbin 17 and the edge 18 are integrally molded. A voice coil 17a is stored in the recessed portion of the voice coil bobbin 17. And, a frame 19 is disposed above and on the plate 15.
Patent Document 1: Japanese Patent Unexamined Publication No. 2005-102166
Patent Document 2: Japanese Patent Unexamined Publication No. H5-30592